Organosiloxane polymers are prepared on an industrial scale using two basic processes. One is condensation which promotes the head to tail condensation of silanol terminated siloxane monomers or oligomers. The other one is equilibration which involves the catalytic rearrangement of siloxane bonds to form an equilibrated mixture. The term equilibration is used to describe the phenomenon which exists when the ratio of linear organosiloxane polymers to cyclic organosiloxane oligomers is maintained at a constant value. During the equilibration process, a constant breaking and forming of siloxane bonds takes place until the equilibrium point is reached. The massive breaking and forming of siloxane bonds permits the use of chainstoppers which will react to form a terminal non-chain extending group on the end of the polysiloxane molecule.
To produce useful silicone polymers by equilibration, the level of trifunctional units has to be very low, often less than 10 ppm. It is very difficult and expensive to produce low trifunctionality containing siloxanes except by a process known as "cracking". In the cracking process, a siloxane mixture containing more than 100 ppm trifunctionality is contacted with KOH at above 135.degree. C. under vacuum to cause the siloxane mass to revert to cyclic siloxanes which distill from the reaction. Siloxanes, generally a hydrolyzate of silicone oil containing linear and cyclic species, are continuously fed to a reactor and are removed at the top of the reactor as cyclic siloxanes. It is well known that the trifunctional units react with the KOH to form a "potassium T salt" which is not volatile and cannot distill with cyclic siloxanes. In this way, trifunctionality is removed from the siloxanes and low trifunctional polymer can be made from cyclics produced by the cracking process. However, some of the trifunctional units remain in the siloxane hydrolyzate in the reactor and the concentration of this trifunctionality continues to increase with time during the cracking process. The buildup of trifunctionality substantially increases the viscosity of the siloxane hydrolyzate in the reactor. The solution becomes thicker and thicker until the reaction is forced to terminate because of the high viscosity. The higher the trifunctionality of the siloxane material fed to the reactor, the faster high viscosity is reached and the shorter the run length. In addition, as the viscosity increases during a run, the rate of "take off" of cyclics decreases, often by 30% or more during the course of the run. At the end of the run, the reactor mass contains so much trifunctionality that the mass is gelled or crosslinked. The reactor has to be shutdown frequently in order to take out the large, hard chunks of gels. These gels have no useful purpose and must be discarded, generally by placing it in a hazardous waster landfill.
U.S. Pat. No. 4,111,973 issued to Bluestein teaches an improved process for increasing the yield and purity of fluoroalkyl cyclotrisiloxanes in a cracking reaction of diorganopolysiloxanes by using, in addition to the cracking catalyst, an effective amount of a higher aliphatic alcohol as a stabilizing additive. This patent specifically directs to the method of making cyclotrisiloxanes. Fluoroalkylsiloxane has very low viscosity in the cracking reactor, thus the siloxane hydrolyzate in the reactor does not gel during the run. The alcohol is added to activate the catalyst rather than to reduce viscosity of the silicone hydrolyzate.
U.S. Pat. No. 4,764,631 issued to Halm et al. provides a method for preparing a product cyclopolydiorganosiloxane via the vapor phase rearrangement of other cyclopolydiorganosiloxanes or mixtures thereof. The vapor phase equilibration essentially eliminates the formation of gel.
U.S. Pat. No. 2,860,152 issued to Fletcher teaches a method of producing cyclic diorganosiloxanes having a composition different from the starting siloxane which comprises heating a mixture of the starting diorganosiloxane and an inert solvent boiling above 250.degree. C. in amount of at least 20% by weight based upon the weight of the siloxane, in the presence of an alkaline catalyst under conditions of temperature and pressure insufficient to cause distillation of the solvent while simultaneously removing the desired cyclic diorganosiloxanes from the reaction zone. The inert solvent shifts the polymer/cyclic equilibrium of the reactor more to cyclics. The more cyclics in the reactor, the less tendency the reaction mass will gel. Fletcher requires at least 20% solvent to delay the gelation. The reaction slows down when too much solvent is put in the system. Further, when the reaction mass gels at the end of a run, the entire reaction content has to be disregarded since the siloxane/solvent cannot be separated from the gel. Thus Fletcher has not completely resolved the problem of gelation.
In none of the references, supra, is there any suggestion or demonstration of a process in which siloxane hydrolyzate containing high trifunctional units can be continuously cracked in the liquid phase without forming the gel.
Accordingly, there is a long-felt need to improve the efficiency of the process for producing cyclic siloxanes in commercial volume at the lowest possible cost.
There is also a need to maximize the yields and rate of formation of cyclic siloxanes from the cracking reaction of the silicone hydrolyzate.
Furthermore, in today's world with substantial ecological concerns it is necessary to eliminate the undesirable gel formed in the cracking process which must be disposed of in an environmentally affective manner.